Visual spatial attention is a critical mental operation that allows us to selectively process only relevant information in the face of an overabundance of visual input. Understanding the underlying neural mechanisms of this phenomenon is central to our understanding of the neural basis of cognition and central to any hope of ameliorating disorders of attention in human patients. Ours and other recent work suggests that better knowledge of the interplay between visual and oculomotor mechanisms may be the key to establishing a causal neural basis of spatial attention. Thus, the long-term goal of this proposal is to understand the reciprocal interactions between oculomotor and vision mechanisms in the primate brain. This goal will be pursued via the following three aims: Our first aim is to test the causal role of frontal cortical saccade mechanisms in directinq covert spatial attention and in drivinq selection in visual cortex. Neuronal activity within the frontal eye field (FEF) will be inactivated pharmacologically and we will examine the effect of this on the ability of monkeys to voluntarily direct attention covertly to the affected part of space, and on the degree to which the widely observed neural correlates of attention in extrastriate area V4 are eliminated. Our second aim is to compare the changes in visual qain observed in extrastriate cortex during FEF microstimulation with the known effects of covert spatial attention. We will examine how the changes in V4 responses caused by subthreshold FEF microstimulation in passively fixating monkeys parallel the modulations observed in animals trained to direct covert attention voluntarily. Our third aim is to examine the relationship between the probability that a saccade will be made to a visual stimulus and the gain of visual cortical responses to that stimulus. We will directly manipulate saccade probability by systematically varying the parameters of FEF microstimulation and study the effect of this manipulation on the gain of visual responses in area V4. This research may provide insight into the physiological basis of disorders of attention in humans, which affect up to 5% of children in the U.S, and may also provide insight into problems of visual-oculomotor coordination, such as dyslexia, which affects approximately 10% of U.S. citizens.